Two-dimensional electron mobility limitation mechanisms inAlxGa1−xN∕GaNheterostructures

Abstract

We analyze electron scattering from phonons, ionized impurities, line dislocations, and interface roughness at an ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{GaN}$ interface. These mechanisms are responsible for mobility limitations in a two-dimensional electron gas. Scattering from charged dislocation lines, ionized impurities, and surface roughness are added to the lattice scattering processes. The dislocation line scattering is described by a new analytical model for the two-dimensional electron scattering. The total mobility variations with temperature, 10--500 K, and electron concentration, $0.05\text{--}2\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, were analyzed. Calculations were compared with available and own experimental data in the temperature range 10--300 K. At temperatures below 50 K the ionized impurity scattering and line dislocation scattering were found to limit the mobility. For temperatures at and above room temperature the optical phonon scattering was predominant. For samples having a rough interface between the AlGaN top layer and the GaN channel there was additional roughness scattering at both high and low temperatures. Therefore, to utilize the heterostructures in field effect transistors, operating at high power levels at or above 300 K, several parameters must be carefully controlled. For a two-dimensional electron gas concentration of $1\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ with mobility above $1500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}∕\mathrm{V}\phantom{\rule{0.2em}{0ex}}\mathrm{s}$ at room temperature, the ionized impurities should be less than $1\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and the line dislocation density below $1\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. Further, the root-mean-square value of the surface roughness need to be less than 1 nm with the lateral surface roughness correlation length larger than 5 nm.